1. Field of the Invention
The present invention relates to an optical displacement sensor using optical means, particularly to an optical encoder.
2. Description of the Related Art
At present, a so-called encoder which is optical or magnetic has been used to detect a linear directional displacement amount in a stage of a machine tool or a three-dimensional measurement instrument, or to detect a rotation angle in a servo motor.
The optical encoder generally includes a scale fixed to a member whose displacement is to be detected, such as a stage, and a sensor head which detects the displacement of the scale. The sensor head includes a light source which irradiates the scale with a light beam, and a photodetector for detecting a diffracted light passed therethrough or reflected by the scale, and detects movement of the scale in accordance with a change of intensity of a received light signal.
FIG. 21 shows a laser encoder in which a coherent light source and diffraction grating scale are used according to a first prior art. FIG. 21 is a constitution diagram showing the laser encoder as one example of a small-sized inexpensive encoder which does not require optical components such as a lens. This laser encoder using the coherent light source and diffraction grating scale is described, for example, in “Jpn. Pat. Appln. KOKAI Publication No. 63-47616”.
That is, as shown in FIG. 21, the laser encoder is constituted such that a transmission type diffraction grating scale 2 is irradiated with a light beam emitted from a semiconductor laser as a surface-emitting laser 1, and specific portions of a diffractive interference pattern 13 generated thereby are passed through transmission slits 53 disposed at a slit pitch p2 and detected by a photodetector 5.
An operation of the first prior art will be described with reference to FIG. 22. In FIG. 22, it is assumed that light receiving areas 4 are formed on photodetectors 3, and the transmission slits 53 and photodetectors 5 are integrally formed.
Here, as shown in FIG. 22, constitution parameters are defined as follows:
z1: a distance between a light source and a plane on which a diffraction grating is formed on the scale;
z2: a distance between the plane on which the diffraction grating is formed on the scale and a light receiving surface of the photodetector;
p1: a pitch of the diffraction gratings on the scale; and
p2: a pitch of a diffractive interference pattern on the light receiving surface of the photodetector.
It is to be noted that the “pitch of the diffraction gratings on the scale” means a spatial period of a pattern which is formed on the scale and whose optical characteristics are modulated.
Moreover, the “pitch of the diffractive interference pattern on the light receiving surface of the photodetector” means the spatial period of an intensity distribution of the diffractive interference pattern generated on the light receiving surface.
Additionally, according to a diffraction theory of light, when z1, z2 defined as described above are in a specific relation satisfying a relation shown in the following equation (1), an intensity pattern analogous to the diffraction grating pattern of the scale is generated on the light receiving surface of the photodetector.                                                         1              z1                        +                          1              z2                                =                      λ                          k              ·                                                (                  p1                  )                                2                                                    ,                            (        1        )            in which λ is a wavelength of the light beam emitted from the light source and k is a natural number.
The intensity pattern which is generated on the light receiving surface and which is analogous to the diffraction grating pattern of the scale is called a Talbot image, and appears in a position which satisfies the above relation equation. This effect is called a Talbot effect.
In this case, the pitch p2 of the diffractive interference pattern on the light receiving surface can be represented by the following equation (2).                     p2        =                  p1          ·                                    z1              +              z2                        z1                                              (        2        )            
When the scale is displaced in a pitch direction of the diffraction grating with respect to the light source, the same spatial period is kept and the intensity distribution of the diffractive interference pattern moves in a direction of displacement of the scale. Therefore, a value of a spatial period p20 of the light receiving areas 4 formed on the photodetectors 3 is set to the same value as that of p2. Then, every time the scale moves by p1 in the pitch direction, a periodic intensity signal is obtained from the photodetector. Therefore, the displacement amount of the scale in the pitch direction can be detected.
Next, FIG. 23 shows the optical encoder according to a second prior art. In FIG. 23, to further miniaturize the first prior art, a light source 1 is disposed on the photodetector 3. The first prior art relates to the transmission type encoder, whereas the second prior art relates to a reflection type encoder. Another constitution is similar to that of the first prior art.
That is, the laser encoder is constituted such that a reflection type diffraction grating scale 2 is irradiated with a laser beam emitted from the semiconductor laser 1 as a coherent light source, and the specific portions of the diffractive interference pattern generated thereby are detected by the photodetector 3.
For the operation of the second prior art, description of a part similar to that of the first prior art is omitted, and only a different part will be described.
For the light beam emitted from the light source 1, the scale 2 is substantially vertically irradiated with a major axis 100 of the light beam, and the light reflected by the scale 2 forms the diffractive interference pattern on the light receiving surface of the photodetector 3. This diffractive interference pattern is similar to the diffraction grating having the pitch p1 on the scale 2 on a condition that positional relation of a light beam emission aperture of the light source 1, light receiving surface of the photodetector 3, and diffractive interference pattern of the scale 2 satisfies the equation (1). The diffractive interference pattern has a period of a pitch p2 enlarged by a magnification calculated by the equation (2).
When the scale 2 is displaced in the pitch direction of the diffraction grating with respect to the light source 1, the same spatial period is kept and the intensity distribution of the diffractive interference pattern moves in the direction of the displacement of the scale. Therefore, every time the scale moves by p1 in the pitch direction, the periodic intensity signal is obtained from the photodetector. Therefore, the displacement amount of the scale in the pitch direction can be detected.
The optical encoder is of a non-contact system with high precision and resolution, and has characteristics such as a superior resistance to an electromagnetic wave trouble. Therefore, the encoder is used in various fields. Particularly in the encoder requiring the high precision and resolution, an optical system is a mainstream.
However, the conventional optical encoder has the following problems.
A first problem is that an output signal from the photodetector is strongly influenced by the diffractive interference pattern incident upon a light receiving area peripheral portion.
In general, in the small-sized optical encoder shown in FIGS. 21 to 23, the photodetector integrated on a semiconductor substrate is used. Moreover, the light receiving portion 4 of the photodetector 3 is disposed only in a region in which the diffractive interference pattern 13 having a sufficient intensity is obtained, and the semiconductor substrate is optically exposed outside the region. Additionally, a main diffracted light from the diffraction grating scale 2 is incident upon the light receiving portion 4, but a part of the light is incident upon a semi-conductor substrate portion other than the light receiving area. The light incident upon the region other than the light receiving portion 4 is a factor for generating an error.
The above-described problem will be described in detail with reference to FIGS. 24A, 24B. FIGS. 24A, 24B show an example of a light receiving area array in which a plurality of light receiving areas are disposed adjacent to one another. When the light is incident upon the photodetector, the light is absorbed by a depletion layer or substrate to generate an electron/hole pair, and is detected as a current.
When the light incident upon the light receiving area 4 is absorbed by the depletion layer in the light receiving area array, the electron/hole pair is generated in the depletion layer. Since an electric field exists in a vertical direction in the depletion layer, the electron/hole pair is taken into a lead electrode of the light receiving area because of the influence of the electric field. That is, the light absorbed in the depletion layer of a certain light receiving area is detected by the light receiving area.
On the other hand, the electron/hole pair by the light incident upon a region in which the light receiving area is not formed in the vicinity of the light receiving area array is absorbed by the light receiving area of the light receiving area array end. As a result, the light is detected as if apparently much light were incident upon the light receiving area of the light receiving area array end.
That is, for the light receiving area of the photodetector, a part of the light incident upon the region in which the light receiving area is not formed is also detected, and a measurement error is generated. Particularly, as shown in FIG. 22, when the light source and the light receiving areas of the photodetector are disposed adjacent to one another, the light beam having a high intensity as compared with the periphery is incident upon the light receiving area end in the vicinity of the light source (coherent light source outer peripheral portion and light receiving area boundary portion) and the problem is serious.
A second problem relates to stability of a light beam intensity of the light source. That is, with the reflection type encoder shown in FIG. 23, a predetermined portion including the major axis of the light beam emitted from the light source, that is, a portion having a highest light intensity is reflected by the scale, and is incident upon the light source in the constitution.
In principle, as shown in FIG. 25, for the diffracted light of the surface-emitting laser 1 diffracted by the diffraction grating, particularly a hatching region in which 0-order and ±1st-order lights interfere generates a strong interference pattern. Moreover, in the reflection type optical encoder shown in FIG. 23, since the diffractive interference pattern by the reflected light moves with the movement of the scale, the light intensity incident upon the emission window of the light source changes with the movement of the scale.
Additionally, in general, it is known that an external light having a high intensity is incident upon the emission window of the light source and then the output intensity of the light beam emitted from the light source is influenced. With the change of the intensity of the external light, the output of the light beam from the light source also changes. When the semiconductor laser such as the surface-emitting laser is used as the light source, the scale functions as an external mirror and forms a complex resonator system, and the influence becomes remarkable.
That is, with the arrangement of the light source in the position shown in FIG. 26, there is a problem that the output intensity of the light beam emitted from the light source changes with the movement of the scale. Therefore, it is possibly difficult to accurately detect the movement of the scale.
Therefore, an object of the present invention is to provide an optical encoder which is miniaturized, which can stabilize the light beam intensity of the light source and the output signal from the photodetector and which is little in detection error and satisfactory in precision.